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Rep. Paul Ryan, the young, charasmatic congressman from Wisconsin who authored the GOP's budget proposal, will fill the bottom half of the Republican presidential ticket -- making the economy and the country's fiscal standing the key campaign issues.

Election 2012: A Look Back

"He understands the fiscal challenges facing America and the fiscal catastrophe that awaits if we don't change course," Romney said of Ryan. "Paul and I are beginning a journey that will take us to every corner of America. We're offering a positive agenda that will lead to widespread prosperity and improve the lives of our fellow citizens."

Wearing a white shirt and a blue blazer with an American flag pin, Ryan took the stage to thunderous applause and quickly began hammering on those same themes.

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"No one disputes President Obama inherited a difficult situation," Ryan said. "And, in his first two years, with his party in complete control of Washington, he passed nearly every item on his agenda. But that didn't make things better. In fact, we find ourselves in a nation facing debt, doubt and despair. This is the worst economic recovery in 70 years."

The Obama campaign quickly responded, issuing a statement that slammed Ryan's fiscal policies -- and highlighted issues that we can expect to hear frequently from Obama in the coming months: Ryan's plans to slash medicare and cut taxes on the wealthy.

"In naming Congressman Paul Ryan, Mitt Romney has chosen a leader of the House Republicans who shares his commitment to the flawed theory that new budget-busting tax cuts for the wealthy, while placing greater burdens on the middle class and seniors, will somehow deliver a stronger economy," Obama campaign manager Jim Messina said in a statement.

Ryan is a rising star in the Republican party, a young, charismatic politician considered the party's budget expert. The seven-term congressman authored the House-backed budget plan that seeks to curb overall entitlement spending and changes Medicare into a voucher-like system to save costs.

Ryan is the first House member to get the VP nod since Geraldine Ferraro in 1984, according to Politico.

His proposed budget was a political lightening rod. The Republican party nearly unanimously backed it -- all but 10 Republican congressman voted for it last year -- but Democrats said it would increase the tax burden on the middle class and savage Medicare. The proposed budget would transform Medicare into a voucher program and lower the top tax rate from 35 percent to 25 percent. It would cut spending by $5.3 trillion over 10 years, and would balance the budget by 2040.

Ryan has long slammed Democrats for failing to adequately address the budget crisis. He returned to that theme Saturday.

"President Obama and too many like him in Washington have refused to make difficult decisions because they're more worried about the next election than the next generation," Ryan said. "We might have been able to get away with that before, but not now. We're running out of time and can't afford four more years."

Earlier this year, speaking to Associated Press news executives, Obama said Ryan's budget was "so far to the right, it makes the Contract With America look like the New Deal."

Ryan will help Romney tighten support among the GOP base who believe the federal government has expanded its reach under President Obama. Through his budget proposals and speeches, Ryan has led the Republican drive to shrink government.

Romney's choice comes as he tries to repair an image damaged by negative Democratic advertising and shift the trajectory of a campaign that's seen him lose ground to President Barack Obama in recent polls.

Romney had one flub in the introduction. When he actually introduced Ryan, he left out one important word: "Join me in welcoming the next president of the United States of America, Paul Ryan."

Romney quickly corrected himself: "Every now and then I'm known to make a mistake," he said. "I did not make a mistake with this guy. But I can tell you this -- he's going to be the next vice president of the United States."

The vice presidential selection will dominate headlines, and Romney's team has been relentlessly teasing the announcement for weeks.

Ryan's selection — as well as Romney's own nomination — will be ratified by delegates to the Republican National Convention that begins on Aug. 27 in Tampa, Fla.